31 March 04 Enrico Fermi and the Miracle of the Two Tables by Gerald Holton There is something quite special about the place of Fermi in history. We all know that in the turbulent flow of time there have arisen, on rare occasions, events that did not fit any previously made plan, but which nevertheless powerfully shaped all subsequent history. Among the most spectacular examples is of course the discovery by a captain, born in Genoa, who set sail toward Asia, but serendipitously encountered instead the land that came to be called the New World. From that moment, the clock for the modern period was set. Another instance of a similar sort was when a then still obscure professor of mathematics at the University of Padua, having used his homemade spyglass for terrestrial explorations, raised it to scan the heavens, and was the first to see there the evidence, in the appearance of the Moon, Jupiter, and Venus, that the existing worldview had to be replaced by a new one. That is when the clock for modern science suddenly came alive. And a third example was a seemingly unplanned event that took place in Rome in October 1934, with transforming consequence--for large sections of physics, chemistry, engineering, medical research, and ultimately for politics and warfare. I refer of course to the discovery, in October 1934, by Enrico Fermi and members of his group, of what was later called the “moderator effect,” the way to turn fast neutrons into slow ones, and the startling new phenomena those neutrons could induce. By itself, such matters would be of interest only to fellow physicists, and ultimately to those good people in Stockholm. But about four years after Fermi’s discovery, in a publication in 2 which Lise Meitner and Otto Frisch immediately recognized the evidence for nuclear fission, its authors, Otto Hahn Hahn and Fritz Strassmann, referred to the key role in their work of "slowed-down neutrons." They did not happen to mention the Italians who had found how to make those slow neutrons. But it can be said that on a day in October 1934, the clock began to tick which ever since has marked the nuclear age in world affairs. I Despite their diversities, these examples, and others of this sort throughout history, have in common not only the initial unintention on the part of the discoverers, and the extraordinary transformations they eventually caused. They also are of the rare sort of research findings which do not correspond to the more usual ones, being not merely discoveries of new facts, or verifications of predictions, or answers to old questions, or support to prop up an unstable theory. They are not just the addition of another brick to the ever-unfinished Temple of Isis. Rather, they suddenly open up access to an area beyond the map of established knowledge, thereby permitting an exploration of a new continent of fruitful ignorance. For what is most prized in science is the discovery of a vast absence of knowledge, of a range of hitherto undiscovered truths, owing to the breakdown of a standard model. Superficially, those examples of profound discoveries might tend to support the view that the course of history itself is decided by the works of great men, to use the title of Wilhelm Ostwald's famous book. That opinion is contrary to the other old illusion, that it is history which shapes the ideas and acts of even the greats. But each of these two 3 opinions is itself illusory. Any study of actual cases soon shows that both mechanisms together are constantly at work. As the psychologist Erik Erikson put it, "an individual life is the accidental coincidences of but one life cycle with but one segment of history." Even the most fateful chance observation has its own prehistory; and conversely, there is no proof that even the most turbulent event in world affairs has been caused by some overarching Zeitgeist. II From childhood on and into his early student years, young Fermi was recognized by his teachers, acquaintances and friends as a prodigy. Relying largely on self-study--a mode typical of great scientists, from Kepler to Faraday to Einstein—he soon became precociously at home with modern physics, enjoying equally the experimental and the theoretical sides. As a very young man Fermi turned to quantum theory, probably the first to do so in Italy, where that subject was considered a sort of no-man's land between physics and mathematics, rather than a promising research site. That part of physics was not taught in Italian universities there, and a dissertation in theoretical physics as such would have been shocking. Therefore, Fermi’s dissertation at the University of Pisa (finished at age 21) had to be an experimental one--on images obtained with monochromatic x-rays by means of a curved crystal. To build the necessary apparatus, Fermi persuaded his fellow students, Franco Rasetti and Nello Carrara, to help him—a first hint of his capacity for organizing teams. Typical also of his later years, Fermi was not satisfied with putting in print the experimental findings (in his seventh published paper, dated 1923). Before that he had 4 published a separate, lengthy theoretical paper on the properties and theory of x-rays. There he showed that he commanded the whole literature—including von Laue, Bragg, Moseley, Barkla, Sommerfeld, Maurice de Broglie, Debye, Scherrer, etc.--in all their several languages. And already then he was keeping physics almost constantly in his thoughts. There is a story, perhaps apocryphal but believable, that one of Fermi's friends once found him pacing up and down in a room, with a preoccupied look. Concerned, his friend asked if Fermi was troubled by something. "No," Fermi replied. "I am just estimating by how much I am depressing the wooden floor as I walk along it." Experimental x-ray studies, and even quantum physics, were by no means the only subjects enchanting the young physicist. It became important for his subsequent career that starting at age 19, Fermi's first five published papers were all on relativity theory. Most of them showed his mastery of the methods of general relativity, the theory just recently confirmed by Eddington's experiment. To be sure, most of the older generation of physicists in Italy were still skeptical and hostile to that theory. But like Wolfgang Pauli and Werner Heisenberg, at about the same time and at the same young age, Fermi had evidently been captivated by Herman Weyl's new book, Raum, Zeit, Materie, for which Einstein himself had written an enthusiastic review in 1918. Fermi contributed to relativity a theorem of permanent value (later called Fermi coordinates), and soon it was incorporated into textbooks on General Relativity. Luckily, Italy had at that time several master mathematicians working in that field, such as Tullio Levi-Civita and Gregorio Ricci-Curbastro. They, and other mathematicians of first rank, including Guido Castelnuovo, Federico Enriques and Vito Volterra, began to notice Fermi's papers, and were ready to support his rise. 5 Yet, again contrary to ordinary expectations, Fermi properly soon realized that relativity theory was not the field in which to build his own career. From 1921 to 1925, he had no less than thirty-one publications, as reproduced in his Collected Papers, most in theoretical physics--even though he knew that there was not a single University chair available for it in all of Italy. His wide-ranging interests and sheer productivity were as astonishing as his self-confidence. Since our theme is the formation of Fermi’s team, I can point out that we have now already met the first member of the group that would soon be formed, the enormously talented experimental physicist and Fermi's schoolmate, Franco Rasetti. It is also time to introduce Orso Mario Corbino, a crucial figure in the eventual rise of Fermi and his group. Twenty-five years older than Fermi, he was widely known for his early work in magneto-optics, for which he had been admired by Augusto Righi of Bologna, considered the previous generation's leading physicist in Italy. After Corbino had been called to the University of Rome, his talent as an administrator and unselfish connoisseur of talent quickly led to his becoming Senator of the Kingdom (1920), Minister of Public Instruction (1921) and Minister of National Economics (appointed in 1923, by Mussolini, although never being a member of the Fascist Party). Corbino's keen scientific mind, combined with his hope to put Italy again on the map as a center of great physics research, led him to mourn the sorry state of physics there, symbolized for him by Righi's death in 1920. He saw clearly that Italy was then unable to take advantage of the world-wide rise of opportunities in the new physics of the day. Without realizing it, by 1920 Corbino was ready to discover a Fermi—just as 6 Enrico Fermi, for his part, must have known that without the help of such a man there might never be a Fermi group. After Fermi's graduation from Pisa, he returned to Rome in 1921, at age 20, living with his parents and his older sister, as he was to do for several more years-- a member of a closely knit family. At the moment, he had neither a job nor prospects for one. But one day he decided to make a first visit to Corbino. The two men immediately took to each other. With Corbino's help, Fermi obtained fellowships, spending unhappy months at Göttingen, and happy ones in Leyden under Paul Ehrenfest, then a couple of years in temporary posts at Florence, working with Rasetti.